The most common and most likely answer to this crossword clue is the 7 plus 5 letter word ROSETTA STONE
I've seen this crossword clue in The Daily Telegraph
R
O
S
E
T
T
A
S
T
O
N
E
More clues leading to the result ROSETTA STONE
- Key to understanding testator one’s unravelled
- Prize for penning university’s key text
- On this, three scripts came up, as in three times one
- Favour including location of Villa’s museum piece
- First clue came up: Dry as air
- First clue from a setter, not so tricky
- Key for translating notes rotates freely
- Slab with key in slate together found outside 24's lodgings
Buddy explains!
The straight part
We know that "museum pie" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case museum pie = rosetta stone.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.